1. Field of the Invention
The present invention relates to ink-jet recording methods and apparatuses used for forming images of input data such as characters, pictures, drawings or the like on a recording medium.
2. Related Background Art
Recently, the ink-jet recording method has been increasingly attracting attention in the field of a data processing, and it comprises the step of forming an image of input data of characters, pictures, drawings or the like on a recording medium (e.g. a sheet of paper, an OHP sheet or a sheet of cloth) by directly ejecting ink droplets from minute nozzles. Therefore, ink-jet recording apparatuses including recording means for performing such method have been used in data processing systems: they have been provided as printers as output terminals of copying machines, facsimiles, printing machines, word processors, work stations, or the like; or they have been provided as handy- or portable-printers of personal computers, host computers, optical disc- or video-equipment or the like.
The recording means (e.g., a recording head) comprises a plurality of recording elements which are arranged in a direction perpendicular to a scanning direction of the recording means. Each recording element has a liquid passage with a recording element (e.g., a thermal element). The liquid passage communicates to a minute nozzle (e.g., an ink ejection outlet) through which ink droplets can be successively ejected to a surface of the recording medium in response to a command corresponding to the input data. The recording element is provided in the liquid passage to generate thermal energy to cause sudden volume change of the ink around the recording element. Consequently an ink droplet is pushed out from the minute nozzle.
In the conventional ink-jet recording apparatus, a certain amount of input data corresponding to one page of the information to be recorded as an image is referred as image data which is divided into several blocks, i.e., the block is an aliquot part of the image data enough to form one line of the page by one scanning movement of the recording head. In this specification, the scanning movement of the recording head is defined as a movement of the recording head for printing one block of the image data so as to form a line of the page in a direction perpendicular to a transporting direction of the recording medium. In this specification, furthermore, the transporting direction of the recording medium is referred as a sub-scanning direction while the direction perpendicular to the transporting direction is referred as a main-scanning direction. Therefore the recording head prints one block of the image data to form one line of the page on the recording medium by performing one scanning movement thereof. By repeating the scanning movements of the recording head, the data corresponding to one page of the information can be recorded on the recording medium. After preparing one block of the image data for the one scanning movement, the recording head starts the scanning movement at a constant speed. During the recording, a plurality of dots is successively putted on a surface of the recording medium by ejecting ink droplets from the recording head. Therefore the block can be defined as a unit of the image data for recording by one scanning movement of the recording head. In general, a size of the block is limited by a capacitive condition of the image memory at a time of directly before the scanning movement, or it is limited by other factors such as the amount of dots to be formed by the recording head of a semimulti-type, a size of the recording medium, a moved distance of the carriage from a starting point, or the like.
In this type of the ink-jet recording apparatus, the recording elements are driven all at once, or block by block consisting of a certain number of the recording elements by applying voltage pulses of a certain width in sequence. In general, it is important to control the pulse width so that each pulse gives just sufficient energy for ejecting ink so that excess energy is not produced. This is important not only for energy saving but also for stabilizing the ink ejection in the course of repetitive drive of the recording head.
In this type of the ink-jet recording apparatus, furthermore, there are two printing modes: a normal mode for directly recording input data just as it is; and a draft mode for recording input data after performing a thinned-out processing in which some dots are removed in accordance with a certain rule or with a certain period for the purpose of: saving the ink; shortening a printing period; or performing a high-speed printing. According to such thinned-out processing, for example, adjacent dots are not printed at the same time so as to a checkered pattern is formed. These modes can be selected by setting the bit switch at a time of switching the electric power on, or they can be selected by operating the key on the operation panel at intervals of forming images corresponding to respective pages of the recording medium.
In spite of the above advantageous features of the conventional ink-jet recording apparatus, however, the conventional recording apparatus cannot select such modes at intervals of forming parts of an image corresponding to respective lines of the page. Therefore a large amount of ink is ejected in spite of that an image of the block having a lot of black dots should be formed, and resulting that a surface of the recording medium becomes wet with the ink and wrinkles easily. In the case of that the draft mode is selected at all times, on the other hand, a small amount of ink is ejected in spite of that an image of the block having a few black dots should be formed, and resulting that an obtained image is hard to read because the characters are blurred.
Furthermore, the conventional ink-jet recording apparatus does not have any means for controlling potencies of the energies used for ejecting ink droplets during a process of recording one block of the image data. Consequently, the recording head can be gradually over heated and a viscosity of the ink to be ejected from the orifices can be gradually decreased when a percentage of the black dots in one block or the prescribed amount of the image data for the one scanning movement of the recording head is comparatively at a higher level (over 50%) during the one block recording. Therefore a volume of the ink droplet tends to rise during the printing and the quality of recorded images can be deteriorated.